It is known in the art relating to overhead cam (OHC) internal combustion engines to provide a direct acting hydraulic valve lifter (DAHVL) that is contacted by a cam and directly actuates one or more valves of the engine. One such arrangement which has been used production engines is shown in U.S. Pat. No. 4,745,888 issued May 24, 1988 to the assignee of the present invention.
In that patent disclosure, a camshaft 18 supported in an aluminum camshaft carrier 11 has cams 22, each of which directly engages a DAHVL (tappet 23) that in turn engages the stem 34 of a poppet valve conventionally carried in a cylinder head, not shown, to actuate the valve. Each lifter 23 includes a cup-like follower having a cam engaging alloy cast iron upper end 24 diffusion bonded to a cold formed steel baffle shell including an annular outer wall (skirt 26) and an inwardly supported central wall 27. The central wall includes a radial supporting baffle and an axial annular cylinder portion in which a hydraulic element assembly (HEA) (hydraulic lash adjuster 28) is reciprocably supported. The HEA is supplied with hydraulic fluid (engine oil) through an annular oil feed chamber 30 which is fed at its lower edge through an opening 32 via an external groove 31.
The follower construction is thin-walled to maintain a low reciprocating weight for the lifter 23 as is desirable for operation at higher engine speeds. However, the chamber 30 is filled with a significant volume of oil which increases the reciprocating mass of the lifter in operation. Also, the oil in the chamber 30 may drain from the lifter when the engine is stopped so that, upon starting, the oil supply must again fill the chamber 30 before a dependable feed of oil is again provided to the HEA 28. During this period, the HEA must rely upon an internal oil reservoir for its oil supply. In addition, air may enter the system such as through draining of the chamber 30 when the engine is stopped or foaming of the oil supply during engine operation. This air may enter the HEA through an inlet from the chamber 30, resulting in unwanted tappet noise and/or improper valve actuation for an extended period until the air is removed from the lifter by escape through the clearances apart from or along with the escaping oil.
Copending commonly assigned U.S. Pat. application Ser. No. 07/610,2454, filed Nov. 8, 1990, now U.S. Pat. No. 5,179,774 issued Jun. 9, 1992, discloses DAHVL's in which oil is displaced from the chamber 30 by oil resistant foam which reduces the operating reciprocating mass of the lifter and supports or assists in supporting the axial cylinder portion of the central wall that in turn supports the HEA. Reference to that patent application, which is incorporated herein by reference, as well as to corresponding applications elsewhere will further explain the background of the additional features included in the present invention.